Subramanian S, Murugesan R, Devasahayam N, Cook J A, Afeworki M, Pohida T, Tschudin R G, Mitchell J B, Krishna M C
Division of Clinical Sciences, National Institutes of Health, Bethesda, Maryland 20892, USA.
J Magn Reson. 1999 Apr;137(2):379-88. doi: 10.1006/jmre.1998.1697.
Design strategies, system configuration, and operation of a dual-channel data acquisition system for a radiofrequency (RF) time-domain electron paramagnetic resonance (EPR) spectrometer/imager operating at 300 MHz are described. This system wasconfigured to incorporate high-speed analog-to-digital conversion (ADC) and summation capabilities with both internal and external triggering via GPIB interface. The sampling rate of the ADC is programmable up to a maximum of 1 GS/s when operating in a dual-channel mode or 2 GS/s when the EPR data are collected in a single-channel mode. By using high-speed flash ADCs, a pipelined 8-bit adder, and a 24-bit accumulator, a repetition rate of 230 kHz is realized to sum FIDs of 4096 points. The record length is programmable up to a maximum of 8K points and a large number of FIDs (2(24)) can be summed without overflow before the data can be transferred to a host computer via GPIB interface for further processing. The data acquisition system can operate in a two-channel (quadrature) receiver mode for the conventional mixing to baseband. For detection using the single-channel mode, the resonance signals around the center frequency of 300 MHz were mixed with a synchronized local oscillator of appropriate frequency leading to an intermediate frequency (IF) which is sampled at a rate of 2 GS/s. Comparison of quadrature-mode and an IF-mode operation for EPR detection is presented by studying the FID signal intensity across a bandwidth of 10 MHz and as a function of transmit RF power. Imaging of large-sized phantoms accommodated in appropriately sized resonators indicates that IF-mode operation can be used to obtain distortion-free images in resonators of size 50 mm diameter and 50 mm length.
描述了一种用于工作在300MHz的射频（RF）时域电子顺磁共振（EPR）光谱仪/成像仪的双通道数据采集系统的设计策略、系统配置和操作。该系统配置为通过GPIB接口结合高速模数转换（ADC）以及内部和外部触发的求和功能。在双通道模式下运行时，ADC的采样率可编程，最高可达1GS/s；在单通道模式下采集EPR数据时，采样率为2GS/s。通过使用高速闪存ADC、流水线8位加法器和24位累加器，实现了230kHz的重复率，以对4096点的自由感应衰减（FID）进行求和。记录长度可编程，最大可达8K点，并且在数据通过GPIB接口传输到主机进行进一步处理之前，可以对大量的FID（(2^{24})）进行求和而不会溢出。数据采集系统可以在双通道（正交）接收器模式下运行，用于传统的混频到基带。对于使用单通道模式的检测，将300MHz中心频率附近的共振信号与适当频率的同步本地振荡器混频，得到一个以2GS/s的速率采样的中频（IF）。通过研究10MHz带宽内的FID信号强度以及作为发射RF功率的函数，对EPR检测的正交模式和IF模式操作进行了比较。对放置在适当尺寸谐振器中的大型体模进行成像表明，IF模式操作可用于在直径50mm、长度50mm的谐振器中获得无失真图像。
